1. Field of the Invention
The invention relates to a method for artificially ageing a catalyst device in accordance with the preamble of claim 1 and to an apparatus which is suitable for carrying out the method.
2. Background
In the past, the US state of California has made a name for itself by imposing early and strict emission legislation for gasoline vehicles. This legislation means that it is necessary to develop exhaust-gas aftertreatment systems which have a service life of more than 100 000 miles. In the European Union, the legislature, using exhaust classification Euro 3, stipulates a service life of 80 000 km, and then from vehicle models from 2005 onward, by introducing exhaust classification Euro 4, a service life of 100 000 km.
To reduce traffic-produced air pollution, in 1988 the California Air Resources Board (CARB) introduced stricter emission limits for all gasoline vehicles in California with OBD (On-Board Diagnosis). In addition to these stricter regulations, all components of relevance in the exhaust gas had to be subject to continuous monitoring by the electronic control units available. The European Union introduced similar legislation for all gasoline vehicles from model year 2001 onward. The exhaust classifications distinguish between the limit values for the emissions and the on-board diagnosis. The limit values for the OBD are provided with a fixed deterioration factor by the CARB and are intended to reflect the performance of an aged component. Therefore, when this exhaust limit is exceeded, the driver must be made aware that a component which is of relevance to emissions is defective by means of an optical system.
This monitoring of all on-board diagnoses must be certified during homologation (approval) before the legislature using correspondingly aged components in order to demonstrate the fault detection. Furthermore, it is necessary to provide evidence that the emission guidelines as prescribed by the legislature are satisfied in accordance with the predetermined durability.
In addition to type certification, the legislature in the field, especially in California and the USA, is monitoring automobile manufacturers to establish whether the prescribed regulations are being complied with and whether the statutory regulations can be extended further. For example, the LEVII legislation, which applies from models dated 2004 onward, primarily provides for stricter regulation of the NOx emissions. This applies in particular to OBD, since currently only the hydrocarbon (HC) emissions are regulated, which means that the automobile manufacturer will also only use diagnosis technology to monitor a deterioration in the conversion of hydrocarbons in the exhaust gas. Since experience and field tests in recent years have established that nitrogen oxide emissions do not have comparable ageing properties to hydrocarbon emissions, from models dated 2004 onward excessive NOx emissions also have to be monitored in addition to the hydrocarbon emissions.
For example, with the introduction of the LEVII legislation in the USA, the emissions from the catalyst to be monitored must not exceed the emission limit to be applied by more than 1.75 times with regard to either the methane-free hydrocarbon emissions or the nitrogen oxide emissions, and the conversion rate of the aged catalyst must not drop below 50% either for the methane-free hydrocarbon emissions or for the nitrogen oxide emissions.
Furthermore, the LEVII legislation requires the ageing of entire exhaust systems for manufacturers which do not provide for cylinders to be shut down in the event of detected misfires above the levels of misfires which damage the catalyst.
The ever-increasing demands imposed by the legislature require automobile manufacturers to invest more research into the ageing processes of components which are of relevance in terms of exhaust emissions.
The aged components represent an important constituent in the investigation of the relevance to exhaust gas during the use of each vehicle. For example, on the one hand it must be established what influence an aged component has on the emissions, and on the other hand it must be established to what extent the performance has changed compared to a new component. This information has to be correspondingly processed in the software during use.
In this context, the ageing performance of catalytic exhaust systems and the simulation of ageing states are of huge importance.
Currently, there are in principle three different artificial ageing methods for motor vehicle catalysts.
One alternative is to simulate misfires in the vehicle. The unburnt air and fuel mix is afterburnt in the catalyst and produces relatively high catalyst temperatures. However, this method of ageing has not proven to be reproducible in the past, since it has been impossible to determine a uniform cycle for ageing the different catalyst systems. Moreover, during a constant operating point it is difficult to keep the temperature in the catalyst at one level. On account of the uncontrolled combustion of the unburnt fuel in the catalyst and the associated uncontrollable temperature peaks, furthermore, the component is weakened, even to the extent of the catalyst being visually destroyed. Furthermore, this ageing method may be relatively time-consuming and cost-extensive, since it generally requires a plurality of catalysts for different tests, and also on account of the expensive use of engine test benches and the availability thereof.
A further method of catalyst ageing is the furnace or vacuum furnace ageing method. In this method, only the monolith of the catalyst is aged at a temperature of from 1000 to 1350° C. in the furnace. After the ageing, the monolith has to be passed on to the corresponding suppliers in order to be installed in the catalyst housing. The correspondingly aged component then has to be mounted in the vehicle and run in for several hundred kilometers in order to be stabilized with regard to emissions. If the emission objective is not achieved after one exhaust gas test, the ageing process has to be repeated. In general, a plurality of iterative ageing steps are required in order to obtain the desired deterioration in the conversion rate. This method can lead to considerable time delays in the OBD application process. In addition to being time-consuming, the work of dismantling and fitting the catalyst and also the stabilizing program which has to be passed through after each ageing step also involve the use of expensive resources. Furthermore, this ageing method cannot be used to age the entire exhaust system, since different loading of the component as a result of the different positions of the catalysts in the vehicle cannot be simulated using this method.
In addition to the methods mentioned above, the artificial ageing of catalysts on the engine test bench is also employed. In one specific long-term running cycle, the catalyst is subjected to extreme temperature peaks and operating states in order to accelerate ageing. In addition, the ageing can be accelerated by lambda controller interventions to set a “rich” or “lean” mix. On account of the use of an engine test bench, this method is relatively expensive and cannot be transferred to different exhaust systems. In this context, it is also possible to use a combination of a rich exhaust-gas mix with additional oxygen enrichment of the exhaust gas for more effective afterburning. However, it has been found that, on account of the temperature peaks which occur, partial melting or even complete erosion of the monolith may occur in the catalyst.
Therefore, all the methods listed are iterative ageing methods which either require a relatively high level of outlay on resources, such as for example engine test benches, or presuppose an unknown number of test carriers. A further problem is that modern catalysts are tailor-made for the individual automobile manufacturers, for example with different cell densities and/or different levels of precious metals, and therefore the ageing characteristics differ between these catalysts. Therefore, it is not currently possible to use a uniform ageing cycle which causes the exhaust gas to deteriorate to a comparable level.